Concerns over the environmental consequences of burning fossil fuels have led to an increasing use of renewable energy generated from sources such as solar and wind. The intermittent and varied nature of such renewable energy sources, however, has made it difficult to fully integrate these energy sources into existing electrical power grids and distribution networks. A solution to this problem has been to employ large-scale electrical energy storage (EES) systems. These systems are widely considered to be an effective approach to improve the reliability, power quality, and economy of renewable energy derived from solar or wind sources.
In addition to facilitating the integration of renewable wind and solar energy, large scale EES systems also may have the potential to provide additional value to electrical grid management, for example: resource and market services at the bulk power system level, such as frequency regulation, spinning reserves, fast ramping capacity, black start capacity, and alternatives for fossil fuel peaking systems; transmission and delivery support by increasing capability of existing assets and deferring grid upgrade investments; micro-grid support; and peak shaving and power shifting.
Among the most promising large-scale EES technologies are redox flow batteries (RFBs). RFBs are special electrochemical systems that can repeatedly store and convert megawatt-hours (MWhs) of electrical energy to chemical energy and chemical energy back to electrical energy when needed. RFBs are well-suited for energy storage because of their ability to tolerate fluctuating power supplies, bear repetitive charge/discharge cycles at maximum rates, initiate charge/discharge cycling at any state of charge, design energy storage capacity and power for a given system independently, deliver long cycle life, and operate safely without fire hazards inherent in some other designs.
In simplified terms, an RFB electrochemical cell is a device capable of either deriving electrical energy from chemical reactions, or facilitating chemical reactions through the introduction of electrical energy. In general, an electrochemical cell includes two half-cells, each having an electrolyte. The two half-cells may use the same electrolyte, or they may use different electrolytes. With the introduction of electrical energy, species from one half-cell lose electrons (oxidation) to their electrode while species from the other half-cell gain electrons (reduction) from their electrode.
Multiple RFB electrochemical cells electrically connected together in series within a common housing are generally referred to as an electrochemical “stack”. One or more stacks electrically connected, assembled, and controlled together in a common container are generally referred to as a “battery”, and multiple batteries electrically connected and controlled together are generally referred to as a “string”. Multiple strings electrically connected and controlled together may be generally referred to as a “site”. Sites may be considered strings on a larger scale.
A common RFB electrochemical cell configuration includes two opposing electrodes separated by an ion exchange membrane or other separator, and two circulating electrolyte solutions, referred to as the “anolyte” and “catholyte”. The energy conversion between electrical energy and chemical potential occurs instantly at the electrodes when the liquid electrolyte begins to flow through the cells.
To meet industrial demands for efficient, flexible, rugged, compact, and reliable large-scale ESS systems with rapid, scalable, and low-cost deployment, there is a need for improved RFB systems.